In making fiberboard according to the wet method from lignocellulose-containing fiber material which has been defibrated in a pressurized, saturated steam atmosphere at temperatures of 100.degree. C. and above, e.g. between 110.degree. C. and 200.degree. C., a certain fraction of the fibrous material is dissolved through hydrolysis of the hemicellulose components among other substances. Depending on the chemical nature of the cellulose-containing material and on the temperature and acidity prevailing during defibration, the amount of dissolved substances may vary between 4% and 12% of the solids in the defibrated material.
In most cases the fibrous material consists of hardwoods and softwoods, but it may also consist to some extent of bagasse, straw etc., which are usually chipped or chopped before defibering. Hereafter, all usable lignocellulose-containing fiber materials will be referred to by the collective term "wood" or, when in comminuted form, by the term "chips".
The amount of the dissolved substances that escapes with the process water and contaminates the environment depends on how effectively the white-water system is closed. If the latter is completely closed, no dissolved substance at all will escape, but all the material will be retained in the finished board. Not only does this afford perfect environmental protection at reasonable cost, but the yield of the fiberboard approaches 100% by weight.
When a fiberboard plant operates with a completely or almost completely closed white-water system, resulting in a high concentration of soluble substances in the white water, an improvement in the mechanical strength and the swelling parameters of the fiberboard is usually obtained, but other qualities such as color, water absorption, and in some cases surface impermeability may be somewhat inferior.
The surface properties of the fiberboard, such as brightness, impermeability and hardness, can be improved and the quality of the board enhanced by applying surface pulp stock of relatively high freeness and slurried in clean or relatively clean water. The concommitant disadvantage, however, is that the forming machine is thereby supplied with extra water that considerably increases the quantity of white water discharge from the machine, something which cannot be allowed in completely or substantially closed white-water systems.